Human APOBEC3 proteins inhibit the replication of human immunodeficiency virus type 1 (HIV-1) via the mutation of viral cDNA during reverse transcription. In sufficient numbers, these mutations lead to the inactivation of proviruses. In patients, APOBECS-mediated restriction is incapable of controlling viral replication because the most potent APOBEC3 proteins are neutralized by the viral accessory protein virion infectivity factor (Vif). Nevertheless, there may be an intermediate scenario in which some APOBEC3 proteins escape degradation and mutate HIV-1 without affecting viral growth. Interestingly, many HIV-1point mutations conferring resistance to antiretroviral drugs occur inAPOBEC3 mutational hotspots, raising the possibility that the virus actually subverts APOBEC3-mediated mutation to enhance its ability to resist treatment. Thus, we hypothesize that sublethal levels of mutation introduced by APOBEC3 proteins contribute to the acquisition of HIV-1 drug resistance. If proven, this will provide a basis for the rational design of antiretroviral regimens that minimize treatment failure by avoiding drugs susceptible to APOBECS-mediated resistance. Such strategies carry the potential to greatly improve the care of people for whom drug abuse complicates treatment and are consistent with "Area of Emphasis 3: Drug Use and HIV/AIDS" of the 2008 funding priorities of the AIDS Research Program at the National Institute on Drug Abuse. To test our hypothesis, we propose two specific aims. Specific Aim 1 - Define the APOBEC3 Proteins Capable of Mutating Vif-proficient HIV-1: These experiments will quantitatively determine which APOBEC3 proteins are present in activated CD4+ T cells and to what extent these proteins are suppressed early in infection. Further experiments will quantify the encapsidation and mutational activity of APOBEC3 proteins in Vif-proficient virions. Specific Aim 2 - Determine Whether APOBEC3 Proteins Impact Drug Resistance: This aim focuses on whether growth of HIV-1in the presence of APOBEC3 proteins affects the acquisition of drug resistance in a cell culture model of infection. Additional experiments will determine whether the depletion of APOBEC3 proteins decreases the ability of HIV-1to develop resistance. PUBLIC HEALTH RELEVANCE: The success of antiviral drugs is tempered by the ability of viruses to acquire mutations conferring resistance to those drugs. These experiments will determine whether natural human proteins known as APOBEC3 enhance the ability of HIV to evolve resistance to the medications used to treat HIV infection. This will have implications for the optimization of anti-HIV therapy.